Serialize and Deserialize N-ary Tree

Description

https://leetcode.com/problems/serialize-and-deserialize-n-ary-tree/

Solution

/*
// Definition for a Node.
class Node {
public:
    int val = NULL;
    vector<Node*> children;

    Node() {}

    Node(int _val, vector<Node*> _children) {
        val = _val;
        children = _children;
    }
};
*/
class Codec {
public:

    // Encodes a tree to a single string.
    string serialize(Node* root) {
        string ret;
        if (root == NULL) return ret;
        helperSerialize(root, ret);
        return ret;
    }
    
    void helperSerialize(Node* root, string& ret) {
        ret += to_string(root->val);
        if (root->children.size() == 0) return;
        ret += '[';
        for(auto& item : root->children) {
            helperSerialize(item, ret);
            ret += ' ';
        }   
        ret += ']';
        return;
    }
    
    int extractNumber(string& s, int& index, int size) {
        int ret = 0;
        while(index < size && (s[index] <= '9' && s[index] >= '0')) {
            ret *= 10;
            ret += (s[index] - '0');
            index += 1;
        }
        return ret;
    }
    
    // Decodes your encoded data to tree.
    Node* deserialize(string data) {
        Node* ret = NULL;
        int index = 0;
        int size = data.size();
        helperDeserialize(data, ret, index, size);
        return ret;
    }
    
    void helperDeserialize(string& data, Node*& root, int& index, int size) {
        if (index >= size) return;
        int val = extractNumber(data, index, size);
        root = new Node(val, vector<Node*>());
        
        if (index == size || data[index] != '[') return;
        index += 1; //Skip the '['
        
        while (index < size && data[index]!= ']') {
            root->children.push_back(NULL);
            helperDeserialize(data, root->children.back(), index, size);
            index += 1; //Skip the whitespace
        }
        index += 1; //Skip the ']'
    }
};

// Your Codec object will be instantiated and called as such:
// Codec codec;
// codec.deserialize(codec.serialize(root));